1. Field of the Invention
The present invention relates to a frequency divider, and, more particularly, to a 2-modulus prescaler which is constructed of a digital circuit so that it can operate at a high speed.
2. Background of the Invention
The digitalization of an RF circuit to be used in a TV receiver, an FM receiver or a mobile telephone has recently been performed. Of these, the development of the frequency synthesizer which operates at radio frequencies in the VHF and UHF bands has been promoted. One of the important parts of such a circuit is a frequency divider which can count down the frequency.
An important consideration for the development of a digital frequency divider is that the division ratio can be varied in accordance with its intended use, i.e., that it can be programmed. Also, it is important that the divider can operate at a high speed and that its production yield is so high that it can be constructed at a low cost.
The digital frequency divider is preferably constructed with a pulse counter using a number of flip-flop circuits. But the construction of the flip-flop circuit operating at a GHz band raises a number of problems in the careful selection of the circuit constituting elements and in the design and production of the device.
As the frequency divider which can operate at a high speed and which can be programmed, the so-called "Swallow-count" frequency divider has conventionally been used. This device is constructed of high and low speed units in which the 2-modulus prescaler developed as the Swallow-count frequency divider is used in the high speed unit. This 2-modulus prescaler can have its number of frequency division (or mode) interchanged in two (or more) stages in response to an external control signal and is used in combination with another programmable (or frequency division number variable) frequency divider which can operate at a low speed.
The Swallow-count frequency divider is, in principle, a variable frequency divider belonging to that class of divider having high operating speeds. However, even so, the device practically has its upper operating limit at 650 MHz at the present time. In case, however, the frequency divider is to be used in a mobile telephone or a UHF TV receiver, there is required a frequency divider which operates at a frequency equal to or higher than about 1 GHz. This requirement has not been satisfied by the conventional frequency divider. Therefore, the realization for the digital frequency divider which can operate at a frequency as high as 1 GHz or more has been desired.